Opthalmic lenses are generally made from blanks furnished with their convex front surfaces already finished, and their back surfaces only partially concave. The concave toric surfaces required under prescriptions for astigmatism are thereafter generated on the back of each blank in compliance with the prescription.
Conventional equipment presently available for such toric concave surface generation is designed with a fixed position for the center of sweeping movement of a rotating cup-shaped cutter past the blank, or vise versa. To change the radius to adjust the surface curvature the motor is moved toward or away from the said fixed center. The net effect of this design condition is to complicate changing the radius to such an extent that it is generally considered necessary for a producer of prescription opthalmic lenses to buy at least two units of concave surface generating equipment to cover the whole range of diopters (about 11/2 to 20) required to serve the market.